The present invention relates in general to electrochemical measuring techniques, and in particular to a new and useful method and device for measuring the energy content of combustion gases.
Energy flow is the important parameter when natural gas is being sold or purchased. The customer is interested in how much heat energy he can get from the gas he is purchasing. The seller typically has a contractual agreement to supply a known minimum energy content with each cubic foot of gas. In order to meet this content the producer typically provides a higher value to compensate for errors in the measurement system. Several parameters or variables must be measured in the present state of measurement technology. Each measurement has its own error contribution. Energy flow is currently not measured directly but is computed by taking an off-line sample to a calorimeter or gas chromatograph for a specific energy measurement. This is used in conjunction with volume flow, temperature, pressure, etc., to calculate an energy flow. This is an involved process, requiring large, expensive instrumentation. This implementation utilizes a tapped fraction of the total flow. This fraction is introduced into a combustion system along with a controlled amount of air. A stoichiometric combustion sensor monitors the output. Its output is used by an element which adjusts the air delivery system. The combustion air flow is measured and the energy flow is calculated from the air flow and the calibration of the split-off fraction or ratio. See Griffis, C. H., et al. "Development of an Accurate Energy Flowmeter", Gas Quality, ed by van Rossum, G. J., Elsevier Science Publishers, Amsterdam, Apr. 22-25, 1986, pages 121-127.
On pages 77, 78 of Heyne, L., "Some Properties and Applications of Zirconia-based Solid-Electrolyte Cells", Proc Interdisp Symposium, 1974, pages 65-88, the use of a combination of an oxygen pump and sensor as an analyzer for combustibles gases is discussed. This depends on combustion at the pump cell. The oxygen introduced is regulated so that it is just what is needed from complete combustion of the combustible gases present. The current or charge used to pump oxygen is thus a measure of the combustible gases.
An apparatus is described in Vizethum, F., Bauer, G., and Tomandl, G., "Computer-Control of Oxygen Partial Pressure", Science and Technology of Zirconia II (Proceedings of the Second International Conference, pages 631 to 635, in which oxygen partial pressure is controlled by a zirconia solid electrolyte cell and an oxygen sensor of the same material. This partial pressure control is used in a gas titration system.
Haaland, D. M., "Internal-Reference Solid-Electrolyte Oxygen Sensor", Analytical Chemistry, Vol. 49, No. 12, October 1977, pages 1813-1817, discusses an electrochemical pumping oxygen sensor with a monitor section and a pumping section. This type essentially uses a small leak from the gas being measured and a pump to evacuate all the oxygen that enters the cell. A measure of the pumping current gives a measure of the oxygen that leaks in and thus a measure of the oxygen in the stream being measured. This type was developed for use in automotive applications.
U.S. Pat. No. 4,841,934 discusses using an oxygen pumping device in conjunction with an oxygen sensor to control the air-fuel ratio of an engine. This uses a combination of an oxygen pumping device and an oxygen sensing device, a double ZrO.sub.2 configuration. This essentially modifies the oxygen concentration operating region of the sensor to improve the signal characteristics of the sensor at its desired operating point. This essentially provides the high sensitivity available at stoichiometry only for other mixtures or concentrations.
Solid oxygen sources are available. These may be metal oxides where the oxygen is liberated upon heating the material. Speidel, R., and Weidlich, E-R, "A solid state oxygen source for UHV", Vacuum, no. 2, 1988, pages 89 to 92, discuss a device for employing CuO as the solid source. Here a constant partial pressure of oxygen is generated by applying heat to the material, causing the material to decompose. The partial pressure is a function of the heat applied to the material. This thus liberates a controlled, small amount of oxygen. This in turn is a self-contained source for the oxygen pump to use as it delivers a controlled, known amount of oxygen to the flow stream inside the energy sensor arrangement.